This application claims priority to Japanese Patent Application No. 2001-370212 filed on Dec. 4, 2001.
1. Field of the Invention
The present invention relates to a signal processing method for an FM-CW radar and, more particularly, to a method for determining whether mispairing has occurred when pairing is done in a situation where there are a plurality of reflecting points from a target. The invention also relates to a signal processing method to be implemented when it is determined that mispairing has occurred.
2. Description of the Related Art
FM-CW radar transmits a triangular-shaped frequency-modulated continuous wave to determine the distance to a target, such as a vehicle, traveling ahead. More specifically, the transmitted wave from the radar is reflected by the vehicle ahead, and a beat signal (radar signal) occurring between the received reflected wave signal and the transmitted signal is obtained. This beat signal is fast Fourier transformed to analyze the frequency. The frequency-analyzed beat signal exhibits a peak at which the power becomes large in correspondence with the target; the frequency corresponding to this peak is called the peak frequency. The peak frequency carries information concerning the distance, and the peak frequency differs between the rising portion and falling portion of the triangular FM-CW wave due to the Doppler effect associated with the relative velocity with respect to the vehicle ahead. The distance and relative velocity with respect to the vehicle ahead are obtained from the peak frequencies in the rising and falling portions. When there is more than one vehicle traveling ahead, a pair of peak frequencies in the rising and falling portions is generated for each vehicle. Forming pairs of peak frequencies in the rising and falling portions is called the pairing.
When the target is a large vehicle such as a truck, a plurality of beams are reflected from the target, and the distances to the respective reflecting points differ depending on their positions on the vehicle. In this case, the distance and the relative velocity must be detected for each reflecting point by pairing the signals between the rising and falling portions based on the peaks occurring due to the beam reflected from the same reflecting point.
However, when a plurality of reflecting points are present, there is a possibility that a peak signal in the rising portion from a certain reflecting point may be erroneously paired, or mispaired, with a peak signal in the falling portion from another reflecting point near that reflecting point, instead of pairing the peak signal in the rising portion with the peak signal in the falling portion from the same reflecting point. Accordingly, an object of the present invention is to provide a method that accurately determines whether or not a mispairing has occurred when pairing is done, and also a method which, if it is determined that a mispairing has occurred, rejects data obtained as a result of the mispairing and thus prevents the data from being used for target computation.
To solve the above problem, according to the FM-CW radar mispairing determination method of the present invention, when a new target is detected, it is determined whether a distance difference between the newly detected target and a previously detected target is within a predetermined range, and when the distance difference is determined as being within the predetermined range, then the difference between the relative velocity of the newly detected target and the relative velocity of the previously detected target is obtained to determine whether the difference is greater than a predetermined value xcex94Va wherein, when the difference is greater than the predetermined value, it is determined that the new target is a target obtained as a result of mispairing.
Further, when the difference between the relative velocity of the newly detected target and the relative velocity of the previously detected target is greater than the predetermined value xcex94Va, then the difference between a distance derivative of the newly detected target over a prescribe time and the relative velocity of the previously detected target is obtained, and when the difference is greater than a predetermined value xcex94Vb which is smaller than the predetermined value xcex94Va, it is determined that the new target is a target obtained as a result of mispairing.
In the above method, it is determined whether the distance difference between the newly detected target and the previously detected target is within the predetermined range but, in addition to the distance difference, a lateral position difference is also compared with a predetermined range to determine if it is within the predetermined range.
Further, in the above method, a time t required for relative distance to change by a prescribed value is obtained based on the relative velocity of the previously detected target, an amount by which the relative distance to the new target has actually changed during the time t from the time the new target was detected is obtained, and the distance derivative is obtained based on the amount of change of the relative distance.
Here, the time t can be obtained by obtaining relative acceleration from the relative velocity of the previously detected target and thereby predicting the relative velocity after a prescribed time, and by applying a correction considering the amount of change of the relative velocity over the prescribed time.
Further, according to the signal processing method of the present invention, when it is determined that a mispairing has occurred, parameters obtained from the target determined as having occurred due to the mispairing are rejected so that the parameters will not be used for combining or control operations.
According to the present invention, whether a mispairing has occurred or not can be determined accurately. If it is determined that a mispairing has occurred, parameters such as relative distance, relative velocity, etc. obtained from the mispairing are rejected and are not used for target computation; as a result, correct data concerning the target can be obtained.